Measurement of size-dependent composition variations for gamma prime (γ') precipitates in an advanced nickel-based superalloy

Energy-dispersive X-ray (EDX) spectroscopy in the scanning transmission electron microscope (STEM) has been used to demonstrate the presence of size-dependent compositional variation for L1 2 -structured Ni 3 Al-type gamma-prime (γ′) precipitates within a commercial RR1000 Ni-based superalloy. This semi-quantitative elemental analysis has been achieved using electrochemical extraction of the γ′ precipitates from the γ matrix. The applicability of this approach to size-dependent compositional analysis of precipitates was confirmed by a comparison of the size distribution for the extracted precipitates with those present in traditional electropolished foil specimens in the size range 20–250 nm. By applying suitable thickness-dependent absorption-corrections we have demonstrated that the composition of γ′ precipitates in our material depends on the size of the precipitate in the range of 5 nm to 3 μm. In particular, the Al content was observed to increase in smaller γ′ precipitates while Ti and Ta contents are constant for all sizes of precipitate. Hf was observed to be present only in the largest precipitates. This type of local compositional information provides invaluable input to assess the accuracy of microstructural modelling for these complex alloys and provides new evidence supporting the importance of anti-site diffusion.